Your Mind and How to Use It: A Manual of Practical Psychology by William Walker Atkinson.

Mind is defined as "the faculty or power whereby thinking creatures, feel, think, and will." This definition is inadequate and circular in nature, but this is unavoidable, for mind can be defined only in its own terms and only by reference to its own processes.

Perhaps the simplest method of conveying the idea of the existence and nature of the mind is that attributed to a celebrated German teacher of psychology who was wont to begin his course by bidding his students think of something, his desk, for example. Then he would say, "Now think of that which thinks about the desk." Then, after a pause, he would add, "This thing which thinks about the desk, and about which you are now thinking, is the subject matter of our study of psychology." The professor could not have said more had he lectured for a month.